Cast aluminum alloy pistons are in widespread use in internal combustion engines. Such aluminum alloy pistons typically are strengthened after casting by a precipitation hardening heat treatment. To this end, the aluminum alloy includes alloying elements, such as Si, Mg, Cu, etc., that form intermetallic and other strengthening precipitates in the alloy microstructure as a result of the post-cast precipitation hardening heat treatment.
In service in an internal combustion engine, different regions of the piston operate at substantially different temperatures. For example, in a particular gasoline engine, the crown of the piston typically attains a temperature of approximately 300 degrees C., which is much hotter than the temperature of the piston boss and skirt (e.g. less than 200 degrees C.). The higher temperature at the crown of the piston subjects the microstructure thereof to what is called overaging whereby the strengthening precipitates present in the microstructure grow in size to an extent that the strength of the crown decreases more rapidly than that of the lower temperature piston boss and skirt. This is undesirable in that, in service, the crown of the piston should have high strength to withstand the combustion stresses at higher service temperatures involved.
It is well known in the art to improve one or more properties (e.g. strength, toughness, wear resistance, fatigue resistance, etc.) of metals and alloys by inclusion of reinforcing particles therein, either as dispersed particles or as one or more preformed inserts formed from particulates. The reinforcing particles can comprise elongated fibers, rounded particles, and other particle shapes. Such reinforced metals or alloys are referred to as metal matrix composites (MMC's) wherein the metal or alloy provides the matrix for the reinforcing particles. Metal matrix composites having various reinforcing particles and matrix metals/alloys are described in U.S. Pat. Nos. ,5679,041 and 5,588,477.
Aluminum based MMC's have been considered as candidate materials for use in selective reinforcement of pistons for internal combustion engines. For example, U.S. Pat. Nos. 4,920,864 and 5,505,171 describe pistons having a piston crown or head having an MMC structure for reinforcement purposes.
An object of the present invention is to provide a cast piston for an internal combustion engine, and method of making same, wherein by selective control of the piston microstructure the mechanical properties of the piston are selectively tailored to different service conditions experienced by different regions of the piston.